Eyonix brings word that Gruul has been hotfixed on the servers, so in case you've been beating your head against the wall trying to down him, now's the time to try again.
The fix consisted of a drop in Gruul's HP, as well as a lowering of the Shatter damage-- Shatter is an AoE Gruul sends out that affects players more or less depending on how close they are to each other. When players are close, it reportedly could do up to 9000 (yes, NINE THOUSSSSAAANNNDD) damage, and if players were able to spread up to 20 yards away, the amount would lower. I haven't done the encounter in the past day, so I'm not sure what it hits for now, but players say it's much easier.
Eyonix also promises that Magtheridon is being inspected by the devs, but there's no word yes as to what changes might come about on that one.